Apparatus for heating and controlling the temperature of a continuously operating cellulose digester



Nov. 23, 1954 J. c. F. c. RICHTER 2,695,232

APPARATUS FOR HEATING AND CONTROLLING THE TEMPERATURE OF A CONTINUOUSLY OPERATING CELLULOSE DIGESTER Filed July 7 1951 H1 5 197 TGRNE rd United States Patent APPARATUS FOR HEATING AND CONTROLLING THE TEMPERATURE OF A CONTINUOUSLY OPERATING CELLULOSE DIGESTER Johan Christotfer Fredrik Carl Richter, Karlstad, Sweden, assignor to Aktiebolaget Kamyr, Karlstad, Sweden, a company of Sweden Application July 7, 1951, Serial No. 235,634

Claims priority, application Sweden July 8, 1950 8 Claims. (Cl. 92--7) The present invention relates to apparatus for heating and controlling the temperature of the contents of a continuously operating cellulose digester.

The invention particularly concerns heating the contents of upright cylindrical digesters which are provided at the top with a device for feeding fibrous material and at the bottom with a device for discharging the digested product. The fibrous material charged into the digester mixes with digesting liquor and continuously and relatively slowly moves from the feeding end to the discharge end while being digested. The digester is constructed with one or more heating zones of limited axial length located intermediate the ends.

The present inventors prior Patent No. 2,414,062, issued January 7, 1947, discloses apparatus for heating the descending column of fibrous material without creating internal turbulence or unduly mixing undigested and partially digested material. This apparatus comprises sieves in the digester wall which are connectable to an outer,

circuit having a pump for drawing ofli digesting liquor, a heater for raising the temperature of such liquor and a. return conduit located substantially in the same transverse plane with the sieve. The return conduit opens into the digester opposite the sieve whereby the returned heated digesting liquor passes essentially crosswise through the fibrous material as it slowly descends through the digester. With this arrangement the d1stances between the return conduit opening and different port-ions of the associated sieve are necessarily unequal; consequently there is a tendency for the heated digesting liquor to pass by the shortest roulte through the fibrous material, with the result that the heating is uneven.

In accordance with the present invention, the inequallty of heating is avoided by providing an inlet for the heated digesting liquor at a point in or near the center of the heating zone. The distances to the different parts of the sieve surrounding a portion or all of the cylindrical d1- gesting wall are equalized, and therefore the flow of heated digesting liquor is more equally distributed, with the result that the heating is substantially even.

The advantages derived from centralizing the inlet of the heated digesting liquid are particularly pronounced in digesters having a comparatively large diameter where it is essential to minimize the heatmg path through the fibrous material. This is necessary in order to pump the heating liquor more readily and to diminlsh the possibility of sucking the fibrous material against the sieve so strongly as to cause it to adhere. In a practical embodiment of the invention, the inlet of the heated liquor is arranged on a tube extending along the substantially longitudinal axis of the digester from one or both ends thereof to the heating zone or zones.

In order that the invention may be more fully understood it will now be described with reference to the accompanying drawings wherein:

. Figure 1 is a view in elevation of an upright cylindrical cellulose digester embodying the heating apparatus of the present invention shown partly in longitudmal section;

Figure 2 is a transverse section of the apparatus in Flgure 1 taken along the line 2-2;

Figure 3 is a longitudinal section of the intermediate portion of the digester of Figure 1 showing an alternate form of the heating apparatus;

2,695,232 Patented Nov. 23, 1954 prising a long cylindrical tank 11 supported in an upright position by means of struts 13 fastened to a foundat-ion 12. At its upper end the digester is provided with two similar motor-driven feeding devices 15 and 15' located symmetrically on either side of the axis of tank '11. Feeding devices 15 and 15 are adapted to supply continuously to the digester a mixture of chips and digest'ing liquor from conduits 17 and 17 in such manner that the tank 11 is always completely filled. Suitable apparatus for this purpose has previously been disclosed by the present inventor in Patent No. 2,459,180, issued January 18, 1949. The feeding devices 15 and 15' are provided with respective feeding screws 19 and 19' surrounded by a sieve 21. A portion of the digesting liquor accompanying the chips passes through the sieve 21 into an annular space 22 between the sieve and the tank wall 23, from where it is drawn off through a conduit 25. The high dilution of the chips in the digesting liquor necessary for their satisfactory introduction into the digester is thus decreased in order that the consistency of the mixture of chips-and liquor be maintained comparatively high, for example, corresponding to a dry content of 15%.

At its lower end the digester is provided with discharging device 27 driven by a motor 28. The discharging device continuously forces digested pulp out of the tank 11 through tube 29. The rate of discharge is so controlled that the pressure in the digester is maintained sufficiently high to prevent vaporization even at the hottest point. During its passage from the feeding end to the discharging end the digester content is heated to the desired digesting temperature by apparatus hereinafter described in detail. The mixture of fibrous material and digesting liquor passes through the digester sufficiently slowly as to permit complete digestion by the time that the material reaches the discharging end. The high concentration of the mixture causes the material to move virtually as a solid column down through the digester without internal turbulence.

A heating zone 30 is located intermediate the ends of the digester usually approximately halfway therebetween. Heating zone 30 is provided with cylindrical vertically-slotted sieves 31 and 32 extending around the entire periphery of tank 11 and forming a screen of the same inner diameter as the adjacent portions of tank 11. A section 33 in the form of an annular blister surrounds sieves 31 and 32. The space between sieves 31 and 32 and section 33 of the digester wall is divided by means of an annular flange 35 into two annular spaces 36 and 37, one above the other, connecting with sieves 31 and 32 respectively. The top edge 38 and bottom edge 39 of section 33 abutting the adjacent narrower portions 41 and 42 of the digester wall are welded in such manner as to form seats for the edges of sieves 31 and 32. Mounted in this manner, the sieves are secured with their inner faces flush with and almost imperceptibly merging with the inner faces of digester wall portions 41 and 42.

A straight tube 43 extends through the upper head of the digester down into the interior thereof along the longitudinal axis of the same and terminates in an open end 44 approximately opposite either sieve 31 or sieve 32. The tube 43 makes a sliding fit with a collar 45 centered on the top of the digester between the feeding devices 15 and 15'; Tube 43 is further supported against lateral movement by means of a spider 46 formed in the plane of the ends of the feeding screws 19 and 19. The lower portion of tube 43 has no further bracing which might offer obstruction or hinderance to the fibrous material as it continuously descends through tank 11. Adjacent the end of tube 43 external to the tank 11, there is fastened a vertical rack 48 engaging a pinion 49, which latter is mounted on supporting structure (not shown) and operated by suitable means such as a crank 51. By rotating crank 51, tube 43 may be moved axially between the position shown in Fig. 1 opposite the middle of sieve 31 to the corresponding position opposite sieve 32.

Outlet connections 52 and 53 are provided on the widened digester wall section 33, opening into spaces 36 and 37, respectively. Conduits 54 and 55 lead from outlet connections 52 and 53, respectively, to a switching valve 56 which serves to connect one or the other of the conduits to a further conduit 57. Conduit 57 leads to a pump 58, the output of which passes through conduit 59 to a heater 61. tube 43 by means of a flexible coupling such as a hose 63.

Pump 58 draws digesting liquor from the heating zone 30, forcing it through heating device 61, wherein it is heated either by heat exchange or by direct introduction of steam into the digesting liquor, and from thence returning it to the heating zone through tube 43 and outlet 44. position as to permit pump 58 to reduce the pressure in space 36 thereby drawing liquor through the vertical slots in sieve 31. The heated digesting liquor is returned to heating zone 30 at the center of the digester opposite sieve 31 and fiows radially outward as shown by the arrows 64 in Fig.'2. The downwardly descending column of fibrous material is thus substantially uniformly heated. After a predetermined time interval, switching valve 56 may be turned to connect conduit 55 to conduit 57, thus causing the digesting liquor to be drawn through lower sieve 32. Preferably, tube 43 is lowered by crank 51 to place its discharging end 44 opposite sieve 32. lf desired. the tube 43 may be permanently positioned halfway between sieves 31 and 32. Under this condition, the heated digesting liquor passe-s to whichever sieve is momentarily connected to pump 58 and heater 61.

Referring now to Fig. 3, there is illustrated an alternative heating zone 30 which may replace heating zone 30 in the structure of Fig. l. An enlarged section 33 of digester wall is provided surrounding cylindrical sieves 31' and 32'. The annular space between sieves 31' and 32' and section 33 is divided into two halves 65 and 66 by means of two vertically and radially directed partitions or flanges 67 and 68 shown in Fig. 4. Spaces 65 and 66 are coupled by means of outlet connections 69 and 71 to conduits 54 and 55, respectively. Depending upon which way valve 56 is turned, the digesting liquor is drawn from the right or left halves of sieves 31 and 32'. The flow of digesting liquor is shown in Fig. 4 by arrows '72 to be generally radially directed over approximately l80 of the circumference of the digester. After a short time interval, valve 56 may be turned to the alternative position and the other half of the cross-section of the digester will receive a substantially even distribution of heated digesting liquor.

The identical tube 43 may be employed with heating zone 30, but a somewhat longer tube 43' having a closed end 73 and apertures 74 in its side walls may be found somewhat more satisfactory. Preferably, tube 43' is perforated along that portion of its length which lies opposite sieves 31 and 32' with result that the digesting liquor is already directed radially upon leaving the tube.

It is to be understood that it is not necessary to provide axial movement of either tube 43 or tube '43 when the digester embodies the heating zone 30. On the other hand, a tube 43' may be advantageously employed with the heating zone 30 keeping in mind that the perforations 74 in tube 43 should extend only the length of one of sieves 3l'and 32. The ability to adjust the axial position of the outlet of tube 43 or 43' is particularly important when cylindrical sieves are spaced a substantial axial distance from each other and used alternately during comparatively long time intervals.

Fig. discloses a form of the digester wherein heating zones 75 and 76 are provided with a substantial axial separation. Zone 75 is formed with a sieve 77 surrounded by an enlarged digester wall section 78 to form an annular space 79 from which the digesting liquor may be drawn by pump 58 when switching valve 56 is in the open position. Heating zone 76 is substantially identical to heating zone 75 and is connected to pump 58 and heater 61 when switching valve 61 is in the alternative position.

A fixed tube 81 extends along the axis of tank 11 through substantially the entire length thereof and is provided with apertures 82 and 83 opposite heating zones In Fig. 1 switching valve 56 is shown in such A conduit 62 connects to the 75 and 76, respectively. Tube 81 is closed by plug 84 intermediate apertures 82 and 83, whereby heated digesting liquor supplied to the top of tube 82 through a conduit 85 is discharged opposite heating zone 75 while heated liquor supplied to the bottom of tube 81 through conduit 86 is discharged opposite heating zone 76.- In order to select the heating zone, a switching valve 87, similar to valve 56, serves to connect conduit 62 to conduit 85 or 86 according to whether valve 56 connects the pump 58 and the heater 61 to heating zone 75 or 76.

I claim:

i. In a vertical cylindrical digester through which fibrous material mixed when digesting liquor is slowly and continuously conveyed longitudinally and wherein the material is heated in a zone short relative to the length of the digester, the combination comprising a sieve arranged in the cylindrical wall of the digester for drawing digesting liquor substantially radially from the heating zone, and a tube extending along approximately the longitudinal axis of the digester through the top end thereof to the heating zone for returning heated liquor to adjacent the center of the heating zone, said tube being supported solely at the top end of said digester.

2. In a vertical cylindrical digester through'which fibrous material mixed with digesting liquor is slowly and continuously conveyed longitudinally and wherein the material is heated in a zone short relative to the length of the digester, the combination comprising a plurality of sieves arranged in the cylindrical wall of the digester and separately operable for drawing liquor from the heating zone, and a vertical cylindrical tube extending through the top end of digester and providing an inlet opening located substantially centrally of the digester in the heating zone for circulating heated liquor to whichever sieve is momentarily operable, said tube being supported solely at the top end of said digester.

3. In a digester according to claim 1, wherein the tube extends substantially the entire length of the heating zone, and longitudinal apertures are formed in the side wall of the tube for returning the heated liquor to the heating zone.

4. In a vertical cylindrical digester through-which fibrous material mixed with digesting liquor is slowly and continuously conveyed longitudinally and wherein the material is heated-in a zone short relative to the length of the digester, the combination comprising a plurality of sieves longitudinally spaced apart in the cylindrical wall of the digester and separately operable for drawing liquor from the heating zone, a tube extending through the top end of the digester substantially along the longitudinal axis thereof and having at least one opening for returning heated liquor to the digester, and means for moving said tube axially for positioning the opening substantially opposite whichever sieve is momentarily operable, said tube being supported solely at the top end of said digester.

5. In a digester according to claim 4 wherein the tube has an open end.

6. In a digester according to claim 4 wherein the tube has vertical slits in the side wall thereof.

7. In a vertical cylindrical digester through which fibrous material mixed with digesting liquor is slowly and continuously conveyed longitudinally and wherein the material is heated in zones-short relative to the length of the digester, the combination comprising a pair of sieves longitudinally spaced apart in the cylindrical wall of the digester and separately operable for drawing liquor from an associated heating zone, a tube extending through the top of the digester substantially the full length of the digester approximately along the longitudinal axis thereof, said tube being longitudinally apertured substantially opposite said sieves, and being supported solely at the top end of said digester, and a transverse partition in said tube between said apertures whereby heated digesting liquor may be separately returned upward through said tube to one heating zone and downward through said tube to the other heating zone.

8. Apparatus for heating and controlling the temperature of a vertical cylindrical cellulose digester, wherein fibrous material and digesting liquor are continuously introduced under pressure at the top end of the digester and digested product is continuously discharged from the bottom end thereof, comprising a sieve formed of longitudinal slits and located in the cylindrical wall of the digester intermediate the ends thereof, a vertical tube extending centrally through the top end of the digester and having longitudinal slits substantially laterally opposite the slits of said sieve, a pumping system for withdrawing liquor from the digester through the slits of said sieve and for reintroducing the liquor in the digester through the slits in said tube, and a heater for heating the withdrawn liquor, said tube being supported solely at the top end of said digester.

Number 5 1,548,477 1,589,837 2,414,062 2,474,862

Number UNITED STATES PATENTS Name Date Morterud Aug. 4, 1925 Cline June 26, 1926 Richter Jan. 7, 1947 Richter July 5, 1949 FOREIGN PATENTS Country Date France Mar. 5, 1940 Norway Dec. 7, 1908 Australia Aug. 28, 1940 

1. IN A VERTICAL CYLINDRICAL DIGESTER THROUGH WHICH FIBROUS MATERIAL MIXED WHEN DIGESTING LIQUOR IS SLOWLY AND CONTINUOUSLY CONVEYED LONGITUDINALLY AND WHEREIN THE MATERIAL IS HEATED IN A ZONE SHORT RELATIVE TO THE LENGTH OF THE DIGESTER, THE COMBINATION COMPRISING A SIEVE ARRANGED IN THE CYLINDRICAL WALL OF THE DIGESTER FOR DRAWING DIGESTING LIQUOR SUBSTANTIALLY RADIALLY FROM THE HEATING ZONE, AND A TUBE EXTENDING ALONG APPROXIMATELY THE LONGITUDINAL AXIS OF THE DIGESTER THROUGH THE TOP END THEREOF TO THE HEATING ZONE FOR RETURNING HEATED LIQUOR TO ADJACENT THE CENTER OF THE HEATING ZONE, SAID TUBE BEING SUPPORTED SOLELY AT THE TOP END OF SAID DIGESTER. 